Wedge light extractor with risers

ABSTRACT

A wedge-shaped light extractor having a light-redirecting surface with a plurality of projections having a riser segment, a plateau segment, and a facet segment, with land segments flanking the projections. The projections improve extraction efficiency, thereby facilitating use of available light from a light source.

TECHNICAL FIELD

This invention relates to electronic displays, and to point to line,line to area or point to area converters for illuminating displays.

BACKGROUND ART

A variety of devices have been proposed for illuminating electronicdisplays. These devices include backlighting panels, front lightingpanels, concentrators, reflectors, structured-surface films and otheroptical devices for redirecting, collimating, distributing or otherwisemanipulating light. Efficient use of the light is particularly importantin battery powered electronic displays such as those used in cellphones, personal digital assistants and laptop computers.

U.S. Pat. No. 4,751,615 (Abrams) shows a tapering wedge transparent pagelighting device. U.S. Pat. No. 4,811,507 (Blanchet) shows a front lightilluminating panel having light-emitting striations whose depthincreases with distance from the light source. U.S. Pat. Nos. 5,005,108(Pristash et al.); 5,050,946 (Hathaway et al.) and 5,594,830 (Winston etal.) show structured-surface panel illuminators whose panel depth variesalong the length of the panel. U.S. Pat. Nos. 4,799,137 (Aho); 4,874,228(Aho et al.); 5,054,885 (Melby); and 5,190,370 (Miller et al.) showvarious light fixtures having inclined or curved prismatic structuredsurface films.

U.S. Pat. Nos. 5,359,691 and 5,608,837 (both to Tai et al.); U.S. Pat.No. 5,485,354 (Ciupke et al.); and U.S. Pat. Nos. 5,608,550 and5,894,539 (both to Epstein et al.) show structured-surface backlightingor front lighting devices having triangular microprisms or microgroovesseparated by flat land areas or lands. European Patent Application EP 0802 446 A1 (Seiko Epson) shows a structured-surface backlighting devicehaving rectangular projections separated by lands. U.S. Pat. No.5,671,994 (Tai et al) shows a structured-surface front lighting devicehaving trapezoidal projections separated by lands.

U.S. Pat. No. 5,396,350 (Beeson et al.); U.S. Pat. Nos. 5,428,468 and5,555,109 (both to Zimmerman et al.) and U.S. Pat. No. 5,555,329 (Kuperet al.) show various structured-surface backlighting devices having anarray of microprisms with tilted sidewalls.

U.S. Pat. Nos. 5,506,929, 5,668,913 and 5,835,661 (all to Tai et al.)and U.S. Pat. No. 5,613,751 (Parker et al.) show light-expanding systemsfor converting a point light source into a linear or planar light beam.

SUMMARY OF INVENTION

Although a number of backlight and front light guide devices have beenproposed, there is an ongoing need for more efficient designs and forreduced power consumption.

Many current designs do not use all of the light supplied by the lightsource. If such unused light could be channeled to the display, thenpower consumption could be further reduced and display brightness couldbe increased.

Several of the above-mentioned designs have a more or less constantthickness and a generally flat structured surface. These designs mayfail to extract light that travels to the end of the device withoutstriking (and thereby being reflected from or extracted by) thestructured surface.

A wedge-shaped design can reduce the amount of (and thus the loss of)light that travels the length of the device without being reflected orextracted. However, some current wedge designs are difficult tomanufacture. For example, if the device tapers to a thin edge at itsdistal end, the edge can be difficult to mold and may fracture or crazein use. If instead a mirrored surface is employed at the distal end,some light will be lost due to inefficient reflection at the mirror, andother light will be lost due to reduced efficiency of extraction forreflected light returning towards the input end of the device. Additionof a mirror can also increase manufacturing costs or lower manufacturingyields.

The efficiency and evenness of light extraction by wedge-shapedbacklight or front light guide devices generally is governed in part bythe gross overall dimensions and shape of the wedge, and by thefine-featured (usually microscopic) dimensions and shapes of anystructured-surface features on the device. For example, the design shownin the above-mentioned Hathaway et al. patent has an inclinedstructured-surface face in the form of a series of descending steps.Each step has a flat land area and an inclined facet. Light typically isextracted from such a device at the facets. Light typically isredistributed within such a device by total internal reflection or “TIR”of light striking the lands or by TIR of light striking an opposingsurface of the device. The reflected light travels toward the narrow ordistal end of the device and may eventually emerge from the device byextraction at a facet. However, some of the reflected light will be lostat the distal end of the device and thus is not available for displayillumination.

Several of the other constant thickness or wedge-shaped backlight orlight guide devices mentioned above can also be regarded as havingfacets and lands, although other terms may have been used to describethe parts of these devices. In general, the overall shape of thebacklight or light guide and the geometry of the structured surface willaffect the ratio of extracted light to reflected light for these devicesas well.

We have found that by using a wedge design having a structured surfacethat includes risers which serve to increase the portion of the facetarea available for light extraction, we can obtain improvements inuniformity of illumination, control over light extraction, efficiency,or in more than one or even all of these factors. The present inventionprovides, in one aspect, a light extraction device having.

a) a light input end;

b) a generally planar light output surface that can deliver light to adisplay; and

c) a structured light-redirecting surface located generally opposite thelight output surface;

wherein the light output surface and light-redirecting surface define agenerally wedge-shaped profile that decreases in thickness from thelight input end towards the center of the device; the light-redirectingsurface reflects light towards the light output surface and thence tothe display; and the light-redirecting surface comprises a plurality ofprojections comprising a riser segment, plateau segment, and facetsegment, with land segments flanking the projections.

Without risers, the amount of extracted light will be determined by theorientation and size of the facets and lands. When risers are added tothe design, the size of each facet (and thus the facet area availablefor extraction) is in effect increased. The overall wedge-shapedextractor profile permits all or a significant portion of the totalfacet area to participate in extraction. Thus, inclusion of a riser anduse of an overall wedge shape enables alteration of the extractor designto increase light extraction to a display. This can enable improvementsin the overall size and efficiency of the device and can make better useof the available light.

The present invention also provides point to line converters, backlightsand front light guides comprising the above-described wedge-shaped lightextraction device, and displays containing such extraction devices.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a is a schematic view of a front light guide.

FIG. 1b is a schematic view of a backlight.

FIG. 2 is a magnified cross-sectional side view of a light extractiondevice of the invention.

FIG. 3 is a magnified cross-sectional side view of a light extractiondevice of the invention, showing descriptive angle β.

FIG. 4 is a magnified cross-sectional side view of a light extractiondevice of the invention, showing descriptive angles ρ, θ, φ, and γ anddescriptive heights r_(h) and f_(h).

FIG. 5 is a perspective view of another light extraction device of theinvention.

FIG. 6 is a perspective view of yet another light extraction device ofthe invention.

FIG. 7 is a perspective view of a further light extraction device of theinvention.

FIG. 8 is a magnified view of the device of FIG. 7, showing the pathstaken by two light rays.

FIG. 9 is a cross-sectional view of an additional light extractiondevice of the invention.

FIG. 10a through 10 e are magnified cross-sectional side views of theextractors in the Examples.

DETAILED DESCRIPTION

The wedge-shaped light extraction devices of the invention provide pointto line, line to area or point to area light conversion that collimatesor redirects light from a light source. The extraction device (which forbrevity will sometimes be referred to as the extractor) can be used toilluminate a display. If the light source is a point light source, thentypically the source has a relatively small cross-sectional areacompared to the cross-sectional area of the display (e.g., on the orderof less than 1/100 of the area of the display). If the light source isan elongated (e.g., a line) light source, then typically the source hasa rectangular cross-sectional area whose long dimension is approximatelyten or more times its short dimension, and which may approximate a shortor long dimension of the face of the display.

Some of the extractor's components and their relationship to one anothercan conveniently be described by comparison to a reference plane.Usually the light output surface of the extractor is planar or nearlyplanar. For purposes of this invention, the reference plane will betaken to be the plane formed by (or closely approximating) the lightoutput surface of the extractor.

Referring now to FIG. 1a, a front light guide and display generallyidentified as 10 a is shown in schematic form. Front light guide 11 alies above display 12 a. Light ray 16 a enters light guide 11 a throughlight input surface 13 a. Light ray 16 a strikes light-redirectingsurface 14 a on a light-reflecting facet segment (not shown), passesdownward as reflected light ray 17 a towards display 12 a, and exitslight guide 11 a through light output surface 15 a. Upon strikingdisplay 12 a (and assuming that display 12 a is suitably modulated toreflect light), light ray 17 a is reflected upward as light ray 18 atowards viewer 19 a. Light ray 18 a passes through surface 15 a which,although identified as a light output surface, also serves in this frontlight guide design as a surface through which reflected light ray 18 acan reenter the light guide), passes through light guide 11 a and exitsthrough light-redirecting surface 14 a at a land, facet or plateausegment (not shown).

Referring to FIG. 1b, a backlight and display generally identified as 10a is shown in schematic form. Front light guide 11 b lies below display12 b. Light ray 16 b enters backlight 11 b through light input surface13 b. Upon striking light-redirecting surface 14 b at a facet segment(not shown), light ray 16 b is reflected upwards as light ray 17 b.Light ray 17 b exits backlight 11 b through light output surface 15 band (assuming that display 12 a is suitably modulated to transmit light)passes through display 12 b towards viewer 19 b.

Referring to FIG. 2, front light guide 20 of the invention has agenerally wedge-shaped cross section that diminishes in thickness fromleft to right in the figure. Light output surface 21 is locatedgenerally opposite a light-redirecting surface having a plurality ofprojections 22 a, 22 b and 22 c and intervening lands such as lands 26and 27. Projection 22 b has riser 23 adjoining plateau 24, which in turnadjoins facet 25. Lands 26 and 27 flank projection 22 b. For purposes ofdiscussion, projections such as projection 22 b can be referred to as“generally trapezoidal”, even though the projection has only three sidesand even though a quadrilateral formed by drawing an imaginary line tocomplete the fourth side of the projection might not have two parallelsides.

FIG. 3 shows front light guide 30 of the invention and an imaginary line36 constructed by drawing a line through vertices 35 a through 35 eformed at the respective intersections of risers 33 a through 33 e withplateaus 34 a through 34 e of projections 32 a through 32 e. Imaginaryline 36 forms an angle β (beta) with a line 37 drawn through the planeformed by generally planar light output surface 31.

Referring to FIG. 4, front light guide 40 of the invention has lightoutput surface 41 and projection 42. Again for purposes of discussion,projection 42 can be referred to as “generally trapezoidal”, even thoughit has only three sides and even though a quadrilateral formed bydrawing an imaginary line to complete the fourth side of projection 42might not have two parallel sides. Imaginary lines 48 a and 48 b aredrawn parallel to light output surface 41 through the respective upperand lower vertices of riser 43. Riser 43 has height r_(h), and formsangle ρ (rho) with imaginary line 48 b (and, if projected, with surface41 ). Plateau 44 slopes downward toward light output surface 41, formingangle θ (theta) with imaginary line 48 a (and, if projected, withsurface 41). Facet 45 has height f_(h) measured between imaginary lines48 c and 48 d which are drawn parallel to surface 41 through therespective lower and upper vertices of facet 45. Facet 45 slopesdownward toward light output surface 41, forming angle φ (phi) withimaginary line 48 d (and, if projected, with surface 41). Lands 46 and47 flank projection 42. Land 47 slopes upward away from light outputsurface 41, forming angle γ (gamma) with imaginary line 48 c (and, ifprojected, with surface 41).

FIG. 5 shows a point to line converter 51 of the invention incorporatinga wedge-shaped extractor. For purposes of illustration, surfacestructures such as projections 52 a, 52 b and 52 c are shown inexaggerated scale. Preferably such projections would be much smaller asdescribed in more detail below. Light rays emitted from point lightsource 50 enter light input surface 54 of beam collector 55 and traveldown the length of light pipe 56. Light rays striking facets such as 55a, 55 b, 55 c and 55 d will be extracted by TIR through light outputsurface 58. Light striking lands such as 57 a, 57 b, 57 c and 57 d willbe reflected by TIR within light pipe 56 towards surface 58 and distalend 59 of point to line converter 56. Upon striking surface 58, theselatter light rays will be further reflected by TIR from surface 58towards other projections (not labeled) in light pipe 56. Substantiallyall the light rays will eventually exit light pipe 56 through generallyplanar light output surface 58 and will form a generally evenlydistributed rectangular light beam.

FIG. 6 shows a point to line converter 61 of the invention having twolight sources 60 a and 60 b. As with the converter of FIG. 5, theprojections in the converter of FIG. 6 are shown in exaggerated scale.Light rays from light sources 60 a and 60 b enter input surfaces 64 aand 64 b of beam collectors 65 a and 65 b and travel down light pipe 66.The thickness of light pipe 66 decreases along the length of the device,reaching a minimum at narrow portion 67. Light is extracted or reflectedfrom projections such as 62 a through 62 d, eventually exiting lightpipe 66 through light output surface 68 as a generally evenlydistributed rectangular light beam.

FIG. 7 shows a front light guide and display assembly 70 of theinvention. Light guide 71 lies atop display 79, separated slightly byoptional air gap 78. Light enters light guide 71 through light inputsurface 76 and travels along the guide to strike facets on projectionssuch as 72 a through 72 d.

FIG. 8 shows a magnified view of a portion of the front light guide 71of FIG. 7. Light ray 81 a strikes facet 75 and is reflected downward aslight ray 81 b through output surface 78 towards display 79. Uponstriking display 79 (and assuming that display 79 is suitablymodulated), light ray 81 b will be reflected upwards as light ray 81 c,reentering light guide 71 through surface 78 and then exiting lightguide 71 through land 76. Light ray 83 a, which approaches thestructured surface of light guide 71 along a different path than lightray 81 a, strikes land 77 and is reflected as light ray 83 b towardssurface 78 and distal end (not shown) of light guide 71. Owing to theslight upward tilt of land 77, ray 83 a will strike land 77 (andreflected ray 83 b will strike surface 78) at a higher angle ofincidence than would be the case if land 77 were parallel to surface 78.For the same reason, reflected ray 83 b will strike surface 78 at apoint located closer to the distal end of light guide 71 than would bethe case if land 77 were parallel to surface 78. This aids in theimproved efficiency of extraction of light from light guide 71, byincreasing the angle of incidence for light eventually reaching thefacets.

FIG. 9 shows the light path traveled by two light rays 91 a and 93 atraveling through a backlight 94 of the invention. Light ray 91 astrikes land 96 and is reflected towards the distal end (not shown) ofbacklight 91 as light ray 91 b. Light ray 93 a strikes facet 95 and isreflected upwards towards display 99 as light ray 93 b. If display 99 issuitably modulated, then light wave 93 b will pass through display 99towards a viewer (not shown).

Overall, the light extractor has a wedge-shaped profile. The verticesbetween the risers and plateaus can if desired lie along a straightline. If the vertices lie along a straight line, and if the light outputsurface is designated as a reference plane, and if an imaginary straightline is drawn through the vertices between risers and plateaus, then theangle β between the reference plane and the imaginary line preferably isabout 0.1 to about 6 degrees, more preferably about 0.4 to about 4degrees. In one embodiment of the invention having riser-plateauvertices that lie along a straight line, the facet heights are constantalong the length of the extractor. In another such embodiment, the facetheights diminish from the input end of the extractor towards the distalor narrow end. In a further embodiment, the facet heights diminish fromthe input end of the extractor towards the distal or narrow end, theplateau segment lengths are constant along the length of the extractor,and the riser heights diminish from the distal end of the extractortowards the input end by an amount sufficient so that the riser-plateauvertices lie along a straight line.

The vertices between risers and plateaus can if desired lie along acurved path. In one such embodiment, the facet heights diminish from theinput end of the extractor towards the distal or narrow end and the landsegment lengths remain constant along the length of the extractor. Inanother such embodiment, the facet heights are constant and the landlengths diminish from the input end of the extractor towards the distalor narrow end. In any event, the angle β for an extractor whose riserplateau vertices lie along a curved path should be approximated bycalculating the arctangent of the (difference between the input end anddistal end thicknesses of the extractor, divided by the length of theextractor), as shown in the following formula:

β=arc tan((input end thickness-distal end thickness)/extractor length).

The individual projections need not all be identical and need not allhave the same angular orientation, shape or dimensions. However, forease of manufacturing, generally it will be preferred to formprojections whose riser, plateau and facet segments have the sameangular orientation and segment length from projection to projection.The land segments also can if desired be similar to one another inangular orientation and segment lengths. Preferably however theprojections are spaced at a relatively coarser pitch near the lightinput end of the extractor, and at a relatively finer pitch towards thenarrowest portion (as in the narrow central isthmus of the point to lineconverter of FIG. 6) or distal end (as in end opposite the light inputend in the extractors of FIGS. 5, 7 and 9 ) of the extractor. Thischange in spacing can conveniently be accomplished by progressivelydecreasing the length of the land segments from the input end to thedistal end (or narrowest portion) of the extractor.

Each riser segment preferably is planar although other shapes such asconvex or concave shapes can be used if desired. The risers do notordinarily reflect or transmit light rays from inside the lightextractor, and thus can have a less than optically smooth surface.However, care should be taken to avoid excessive riser surfaceroughness. That will help avoid ghosting and other visual artifacts thatmay arise due to backscattering of light within the light extractor andconsequent weak transmission of backscattered light through the risers.The riser heights r_(h) preferably are about 0.001 to about 0.5 mm, morepreferably about 0.002 to about 0.02 mm. The angle ρ between thereference plane and a riser preferably is about 90 to about 45 degrees,more preferably about 85 to about 65 degrees.

Usually a portion of each plateau segment will lie in the shadow cast bylight rays striking the adjoining riser and thus will not participate inlight reflection or extraction. In a front light guide construction, theunshadowed portions of the plateaus can serve as upper viewing ports forextracted light. Preferably, the plateaus are planar although they canhave other shapes such as convex or concave shapes if desired.Preferably however the plateaus have flat, optically smooth surfaces soas not to induce distortion in the extracted light. The plateau segmentlengths preferably are about 0.001 to about 1.5 mm, more preferablyabout 0.004 to about 0.12 mm. The angle θ between the reference planeand a plateau preferably is about 0 to about 10 degrees, more preferablyabout 0 to about 2 degrees, with the plateau segments most preferablybeing parallel to the reference plane (or in other words, θ mostpreferably is zero).

The facet or ramp segments direct light toward the display whereupon itcan be extracted (sometimes following one or more further reflections)and suitably modulated by the display. Preferably, the facets are planaralthough they can have other shapes such as convex or concave shapes ifdesired. Preferably the facets have flat, optically smooth surfaces soas not to induce scattering of the reflected light. The facet heightsf_(h) preferably are greater than the riser heights, and more preferablyare about 0.001 to about 1.5 mm, most preferably about 0.002 to about0.12 mm. The angle φ between the reference plane and a facet preferablyis about 20 to about 50 degrees, more preferably about 25 to about 45degrees.

The land segments of the light-redirecting surface reflect light by TIRtoward the narrow portion or distal end of the extractor. The lands canalso serve as a lower viewing port for extracted light. Preferably, thelands are planar although they can have other shapes such as convex orconcave shapes if desired. Preferably the lands have flat, opticallysmooth surfaces so as not to induce scattering of the reflected light.As noted above, the land segments preferably decrease in length alongthe length of the extractor. A preferred pitch is from about 0.06 toabout 12 projections per mm at the light input end to about 250 to about1 projection per mm at the distal end or narrowest portion of theextractor. A preferred land segment length is about 0.003 to about 15mm, more preferably about 0.003 to about 1.2 mm. The angle γ between thereference plane and a land preferably is about 0 to about 5 degrees,more preferably about 0.5 to about 2 degrees. Most preferably, the landsare tilted in the opposite direction from the wedge. When so tilted,light that comes from the input end of the extractor and strikes thelands will tend subsequently to strike the facets at a higher angle ofincidence, thereby increasing extraction efficiency.

The projections can extend across the full width of thelight-redirecting surface (as is shown, for example, in FIG. 7) or canbe in the form of shorter, less than full width segments, which can bealigned with one another in rows and columns or staggered from row torow. Rows of projections can be arranged in parallel to the light inputsurface or at an angle with respect to that surface. Preferably, theprojections extend across the full width of the light-redirectingsurface and are generally parallel to the light input surface.

The light input surface and light output surface can be planar or have aconvex or concave curvature. When the light source is a point source,the light input surface preferably has a convex curvature. If theincoming light is not evenly distributed along the width of the inputsurface, then lenslets, prisms, a roughened surface or othermicrostructures can be added to the light input surface to more evenlydistribute the light. The light output surface preferably has anoptically smooth finish, in order to minimize transmission losses,undesired scattering and distortion.

The light extractor can have any desired overall size and thickness butpreferably it is as thin as possible, e.g., 5 mm or less at the thickestportion of the extractor. The light extractor can be square,rectangular, oval or any other desired shape when viewed from theintended display observation point. The size and shape of the extractorusually will be dictated by the size and shape of the desired displaydevice and by whether the extractor is intended to function as a pointto line, line to area or point to area converter. The light extractorsof the invention are particularly useful as point to area or line toarea front light guides for subminiature or miniature display devicesilluminated with light emitting diodes (LEDs) powered by smallbatteries. Suitable display devices include color or monochrome LCDdevices for cell phones, pagers, personal digital assistants, clocks,watches, calculators, laptop computers, vehicular displays and the like.The display can also be a fixed graphic such as a poster or sign, or avariable-appearance substrate such as “Gyricon” electronic displaymaterial (under development by Xerox Corporation). In addition to LEDs,other suitable illumination sources include fluorescent or incandescentlamps, electroluminescent lights and the like.

The light extractors of the invention can be fabricated from a widevariety of optically suitable materials including polycarbonate;polyacrylates such as polymethyl methacrylate; polystyrene and glass,with high refractive index plastics such as polycarbonate beingpreferred. The light extractors preferably are made by molding,embossing, curing or otherwise forming an injection moldable resinagainst a lathe-turned tool or other formed surface, made of metal orother durable material that bears a negative replica of the desiredstructured surface. Methods for making such formed surfaces and formolding, embossing or curing the extractor will be familiar to thoseskilled in the art. Individual light extractor designs can if desired beevaluated without the need for actual extractor fabrication, by usingsuitable ray-tracing modeling software such as “ASAP” from BreaultResearch Organization, Inc., “Code V” and “Light Tools” from OpticalResearch Associates, “Optica” from Wolfram Research, Inc. and “ZEMAX”from Focus Software, Inc.

The invention is further illustrated in the following non-limitingexamples.

EXAMPLES

Two generally planar and three generally wedge-shaped extractors weresimulated using CAD software and evaluated using ASAP ray tracingsoftware. This approach gives generally good agreement with observedactual results without the need to mold and fabricate prototypes. Eachextractor would be made from 1.58 refractive index polycarbonateplastic. All would have a 30 mm width, 40 mm length, and 1 mm thickinput aperture. The distal ends of Extractors 1 and 2 would be 1 mmthick. The distal ends of Extractors 3, 4 and 5 would be 0.2 mm thick.The light sources were assumed to be rectangular, 30 mm long, 1 mm high,and with a Lambertian light distribution.

The general shapes of the steps or projections in Extractors 1 through 5are shown in FIGS. 10a through 10 e, respectively. The dashed lines inFIGS. 10a through 10 e are drawn parallel to the reference plane.Extractors 4 and 5 would be extractors of the invention. The angles anddimensions of the segments for each extractor are listed below in TableI. For Extractors 1 through 4, the lands (and plateaus if present) wouldbe horizontal (γ is zero). For Extractor 5, the lands would tiltslightly upward at an angle γ of 0.75 degrees. For all extractors, therisers (if present) would be tilted at an angle ρ of 90 degrees. Thefacets in each extractor would be tilted at an angle φ of 30 degrees.

For Extractors 1 through 3, the riser height and facet height valuesshown in Table I would apply to the entire extractor. For Extractors 4and 5, Table I shows the height of the facet nearest the input end ofthe extractor. The facet heights for Extractors 4 and 5 would decreasefrom the input end towards the distal end of each extractor. ForExtractor 5, Table I shows the height of the riser nearest the distalend of the extractor. The riser heights for Extractor 5 would decreasefrom the distal end towards the input end of the extractor. Thesevariations in facet or riser height permit the riser-plateau vertices ofExtractors 4 and 5 to lie along a straight line.

The pitch between projections was varied for each extractor in order tooptimize predicted light output to a display to within about ±10% overthe area of the extractor's light output surface. The predicted lightextraction efficiency for each extractor was then calculated andrecorded.

Extractors 1 and 3 through 5 were assumed to have 200 steps orprojections along the length of the extractor. Extractor 2 was assumedto have 400 projections along the length of the extractor. This providedan increase in efficiency compared to Extractor 2, but uniformity oflight output decreased and the large number of projections would makeExtractor 2 more difficult to manufacture.

TABLE I Riser Plateau Facet Total No. Starting Minimum Extractor Height,Length, Height, of Pitch, □Pitch, Pitch, Extractor No. β γ ρ θ φ mm mmmm Projections mm mm mm Efficiency 1 0° 0° 90° 0° 30° 0.005 0.017680.005 200 0.30 1.00363 × 0.06 27% 10⁻³ 2 0° 0° 90° 0° 30° 0.005 0.017680.005 400 0.175 4.151 × 0.06 42% 10⁻⁴ 3 1.45° 0° NA 0° 30° NA NA 0.004200 0.21832 0 0.21832 40% 4 1.45° 0° 90° 0° 30° 0.005 0.01768 0.013 2000.4 2.077855 × 0.06 56% 10⁻³ 5 1.45° 0.75° 90° 0° 30°  0.0046 0.017680.0138 200 0.44 2.56236 × 0.06 59% 10⁻³ Notes: “NA” = Not Applicable

As shown in Table I, the extractors of the invention (Extractor 4 andExtractor 5) had a considerably better overall efficiency thanextractors without risers (Extractor 1 and Extractor 2). The extractorsof the invention also had considerably better overall efficiency than astep-wedge style extractor (Extractor 3). This improved efficiencyshould help provide significantly increased battery life in abattery-powered display.

Further increases in efficiency may be available via further adjustmentof the land tilt angle γ but at a potential decrease in overalluniformity of illumination.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention. It should be understood that this invention is notlimited to the illustrative embodiments set forth above.

What is claimed is:
 1. A light extraction device having: a) a lightinput end; b) a generally planar light output surface that can deliverlight to a display; and c) a structured light-redirecting surfacelocated generally opposite the light output surface; wherein the lightoutput surface and light-redirecting surface define a generallywedge-shaped profile that decreases in thickness from the light inputend towards the center of the device; the light-redirecting surfacereflects light towards the light output surface and thence to thedisplay; and the light-redirecting surface comprises a plurality ofprojections comprising a riser segment, plateau segment, and facetsegment, with land segments flanking the projections.
 2. A lightextraction device according to claim 1, wherein the presence of theriser segments increases the extraction of light from the device by thefacet segments.
 3. A light extraction device according to claim 1,wherein variation in the height of the riser segments will alter theratio of light that is reflected by the facet segments to light that isreflected by the land segments.
 4. A light extraction device accordingto claim 1, wherein if an imaginary line is drawn from the vertexbetween a land segment and an adjoining riser segment on a projection tothe vertex between a facet segment on that projection and an adjoiningland segment, in order to form an imaginary quadrilateral, thequadrilateral is generally trapezoidal in cross-section.
 5. A lightextraction device according to claim 1, wherein the device decreases inthickness from the light input end along the entire length of thedevice.
 6. A light extraction device having: a) a light input end; b) agenerally planar light output surface that can deliver light to adisplay; and c) a structured light-redirecting surface located generallyopposite the light output surface; wherein the light output surface andlight-redirecting surface define a generally wedge-shaped profile thatdecreases in thickness from the light input end towards the center ofthe device; the light-redirecting surface reflects light towards thelight output surface and thence to the display; and thelight-redirecting surface comprises a plurality of projectionscomprising a riser segment, plateau segment, and facet segment, withland segments flanking the projections, wherein if an imaginary line isdrawn through the vertices between riser segments and adjoining plateausegments, and the light output surface is designated as a referenceplane, then the angle β between the reference plane and the imaginaryline is about 0.4 to about 4 degrees.
 7. A light extraction deviceaccording to claim 6, wherein the angle ρ between the reference planeand a riser segment is about 85 to about 65 degrees, the angle θ betweenthe reference plane and a plateau segment is about 0 about 2 degrees andthe angle φ between the reference plane and a facet segment is about 20to about 60 degrees.
 8. A light extraction device according to claim 6,wherein the angle γ between the reference plane and a land segment isabout 0.5 to about 2 degrees.
 9. A light extraction device according toclaim 6, wherein the land segments and the imaginary line are tilted inopposite directions with respect to the reference plane.
 10. A lightextraction device according to claim 6, wherein the vertices lie along astraight line.
 11. A light extraction device according to claim 6,wherein the vertices lie along a curved line.
 12. A light extractiondevice according to claim 1, wherein the lengths of the land segmentsdiminish from the input end of the extraction device towards thenarrowest portion or distal end of the extraction device.
 13. A lightextraction device according to claim 1, wherein the projections arespaced at a relatively coarser pitch near the light input end of theextraction device, and at a relatively finer pitch towards the narrowestportion or distal end of the extraction device.
 14. A light extractiondevice according to claim 1, wherein the riser segment heights r_(h) areabout 0.001 to about 0.5 mm, the plateau segment lengths are about 0.002to about 1.5 mm, the facet segment heights f_(h) are about 0.001 toabout 1.5 mm and the land segment lengths are about 0.003 to about 15mm.
 15. A light extraction device according to claim 14, wherein thefacet segment heights are greater than the riser segment heights.
 16. Alight extraction device according to claim 1, wherein the projectionsextend across the full width of the light-redirecting surface and aregenerally parallel to the light input end.
 17. A display devicecomprising a point or elongated light source optically coupled to alight extraction device according to claim 1, the extraction devicebeing optically coupled to a light valve that modulates light from thelight extraction device.
 18. A display device according to claim 17,wherein the light extraction device is a front light guide.
 19. Adisplay device according to claim 17, wherein the light extractiondevice is a backlight.
 20. A display device according to claim 17,wherein the display device comprises a cellular telephone, pager,personal digital assistant, clock, watch, calculator, laptop computer ortransportation vehicle.